In the past, ultrasonic diagnostic apparatuses which give a drive pulse from a drive circuit to an ultrasonic transducer unit, transmit ultrasonic wave from the ultrasonic transducer unit to a target region, receive an echo signal from the target region by the ultrasonic transducer unit, perform processing such as amplification, addition, and logarithmic compression to obtain image data, transmit the image data to a monitor, and thereby display a desired diagnostic image on the monitor have been disclosed (for example, refer to the following Patent Document 1).
Further, ultrasonic signal processing apparatuses which, in processing an echo signal, convert an analog receiving signal received by an ultrasonic transducer unit to a digital signal, multiply the digital signal by a reference signal in a given frequency, and thereby convert an signal waveform thereof have been disclosed (for example, refer to the following Patent Document 2). Furthermore, ultrasonic diagnostic apparatuses which convert an analog receiving signal to a digital signal, synthesize the obtained digital data, and thereby obtain an ultrasonic tomogram have been disclosed (for example, refer to the following Patent Document 3).
FIG. 6 is a block diagram showing a schematic structure of an ultrasonic wave transmitting/receiving section and a signal conversion section in the foregoing ultrasonic signal processing apparatus and the foregoing ultrasonic diagnostic apparatus which convert an analog receiving signal to a digital signal. The transmitting/receiving section and the signal conversion section include an ultrasonic transducer unit 1 for transmitting ultrasonic wave and receiving an echo signal, a drive circuit 2 for transmitting a drive pulse to the ultrasonic transducer unit 1 according to a trigger signal, and an A/D converter 4 for converting the echo signal received by the ultrasonic transducer unit 1 to a digital signal and outputting the resultant signal.
In this case, the echo signal received by the ultrasonic transducer unit 1 is a large amplitude signal in a superficial part and is a weak signal in a deep part. In result, in some cases, a resolution (bit number) of the A/D converter 4 falls short (bit number is small). Therefore, in actual ultrasonic diagnostic apparatuses, an unshown preamplifier and an unshown variable gain amplifier (VCA) are connected between the ultrasonic transducer unit 1 and the A/D converter 4. Thereby, the amplitude of the echo signal is controlled, a dynamic range thereof is decreased, and then a resultant signal is inputted to the A/D converter 4.    Patent Document 1: Japanese Patent Application Publication No. 6-154210 (paragraph 0011, FIG. 1)    Patent Document 2: Japanese Patent Application Publication No. 6-313764 (paragraph 0008, FIG. 1)    Patent Document 3: Japanese Patent Application Publication No. 7-171152 (paragraph 0023, FIG. 1)
However, in the foregoing ultrasonic signal processing apparatus and the foregoing ultrasonic diagnostic apparatus, immediately after the drive circuit 2 transmits a large amplitude pulse to drive the ultrasonic transducer unit 1, ringing having a low frequency component is caused by influences of an piezoelectric device and a cable which compose the ultrasonic transducer unit 1. In result, as shown in FIG. 7, ringing of a large amplitude pulse is overlaid on an echo signal of a superficial part obtained immediately after driven by a transmission pulse. Therefore, to sufficiently input the echo signal of the superficial part to the A/D converter 4 within the input upper/lower limit range indicated by an arrow, it is necessary to use the A/D converter 4 with a large bit number so that gain is lowered in the superficial part. However, the resolution of the A/D converter capable of being practically realized inexpensively is about 12 bits. By using such an A/D converter, only a dynamic range of about 60 dB can be handled. Further, a gain variable range of a variable gain amplifier is about 40 to 60 dB. Therefore, for example, to handle an echo signal on which ringing of 120 dB is overlaid, the bit number falls short. The foregoing ultrasonic signal processing apparatus and the foregoing ultrasonic diagnostic apparatus have been developed to resolve such a problem. However, in the foregoing ultrasonic signal processing apparatus and the foregoing ultrasonic diagnostic apparatus, there have been problems that the structure becomes complex, and the apparatus becomes large and expensive.